Casting is a process of forming a structure by introducing a fluid material into a cavity or mold in which the material is allowed to solidify. The shape of the cast structure is determined by the configuration of the mold. Many materials are cast including metals, plastics, rubber and cement.
Heretofore, casting of cementitious materials was limited to a gravity feed method of vertically pouring the cementitious mix into a form or mold which defines a pattern for establishing the exterior or interior configuration of the cast structure. The form or mold may be made from wood, ceramic, metal, plastic or other resilient materials including latex, rubber or silicone. The admixture cures in the mold with a surface configuration conforming to the surface configuration of the mold.
The mold for a cast cement structure in addition to defining the shape of the cast is also used, at present, to hold the cementitious mix in a stationary position until the initial set has occurred. During the pouring operation, the mold is vibrated to aid in uniformly distributing the mix within the mold and to eliminate entrapped air. Agitation of the mold is known to cause migration of heavy aggregate to the bottom of the mold and separation of lighter particles to the top. The more the uncured material is agitated, the greater the segregation. The fluidity of the mix based on the water added will also contribute to segregation. Accordingly, following conventional practice, some segregation is unavoidable and basically a tradeoff to achieve relative homogeneity and to eliminate entrapped air.
The cementitious mixtures of Portland cement and aggregate presently used for casting have a high slump value to provide good flow characteristics. Such cement mixtures also exhibit a high degree of dilatency. The degree to which the mix gives off free water is a measure of the dilatency of the admixture. Dilatency results in an alternation in the water cement ratio which, in turn, affects the cure properties of the mix. The degree of segregation and latence increase with agitation and increased slump. Thus, the physical properties of the cured concrete may vary from area to area within the casting.
Once the mold is filled and leveled, conventional practice requires a quiescent period of immobility. Using conventional high slump value Portland-aggregate mixes, the mix must be held immobile to cure properly without cracking. Otherwise, the cast product will be flawed.
For all the above reasons, present day molded cast products have limited utility and generally exhibit poor surface finish and high porosity. Moreover, it is difficult using conventional cement casting techniques to cast a single molded cement structure of relatively unlimited length. The size of the structure is also a constraint.
In a companion patent application, U.S. Ser. No. 783,868, filed Oct. 3, 1985, now U.S. Pat. No. 4,666,521 the disclosure of which is herewith incorporated by reference, a cementitious admixture is described comprising hydraulic cement, presized inorganic filler particles and water mixed in a predetermined proportion to form a thixotropic nondilatent cementitious composition. Thixotropy develops when dilatency is minimal so that flow will occur only when the mix is subjected to a shearing force. It has been found in accordance with the present invention that using a cementitious admixture which is substantially non-dilatent in a mold which is rotated will provide a product with a high surface finish less porosity and of a substantially greater density than conventionally formed cement cast structures. Ornamental concrete designs and concrete pipe or columns can be cast using the method of the present invention. By rotating the mold rotational forces cause displacement of the material to the outermost walls of the cavity of the mold. Segregation of the particles is avoided by the use of an admixture which is thixotropic. The technique disclosed by the present invention permits castings to be made of any length as long as the mandrel or rotational fixture can support the weight of the casting and as long as the mixture can be inserted into the mold hollow core.